The process of the present invention relates to a "post-press" or second press operation for creating thick, hard, integral surface portions on at least one and preferably both surfaces of a man-made fiberboard substrate. A handleable mat is first manufactured by consolidating a cellulosic fiber-containing composition to form a consolidated fiberboard having a density of less than 35 pounds per cubic foot. The handleable mat can be consolidated by drying a mat formed by depositing a water slurry of fibers onto a water pervious support member, as is common in the manufacture of insulation board, or the mat can be consolidated under heat and pressure until substantially dry in a first press operation. This consolidated mat is thereafter "post-pressed" to form dense surface layers for stiffness and strength, while retaining the relatively low density core or central portion. The density of the core is substantially predetermined when the mat is initially consolidated. For the purpose of the present invention, a "post-press" operation refers to a press step performed on the handleable mat after consolidation thereof.
It is necessary to form the fiberboard of the present invention in two separate operations. The first operation forms a consolidated mat and sets the core density while the second or "post-press" operation creates the denser surface portions. It is quite surprising that a "post-press" or second pressing operation is effective in densifying a surface of a consolidated mat while leaving the density of the core material substantially unchanged. Prior to the present invention, it was thought that the qualities and physical characteristics of a board would be finally determined by the conditions under which the board is initially consolidated and by the raw materials used to form the board, because of the bonding occurring during consolidation.
Much of the bonding necessary for cohesion and strength in a man-made board occurs during the consolidation of the board during hot-pressing. Prior to drying a board made by the wet process, the board is weak, but the bonding forces created during the final consolidation are generally quite powerful. Generally the same bonding effect occurs in the final consolidation of a board made by the dry process. Prior to hot-pressing, the loosely disassociated cellulosic fibrous product is quite weak, but after hot-pressing the mat into its final configuration, it is very powerfully held together by the bonding which occurs during hot-pressing. Hot-pressing during consolidation causes a welding or coalescing of the cellulose fibers at the surface of the product so that the surface portion consists of wood remade in modified form. It is very difficult to densify or restructure the surface of a consolidated product without again destroying these fiber-to-fiber surface welds. It is especially difficult to densify only a surface portion of a consolidated board in an amount sufficient to substantially increase the strength and stiffness of the product while retaining the lightweight core material to provide a lightweight product. The process described herein accomplishes these results.
A prior application, assigned to Masonite, Ser. No. 537,254 filed Aug. 23, 1974, now U.S. Pat. No. 4,007,076 discloses and claims a method of post-press embossing consolidated, man-made boards to effectively and accurately reshape the surface of a completed panel without surface defects such as surface fiber tearing. In accordance with Ser. No. 537,254 filed Aug. 23, 1974, a previously consolidated panel is post-press embossed at a pressure of at least 1000 psi to create a surface design, or to permanently redesign the surface of a consolidated product without destroying the interfiber bonding established at the surface of the board during consolidation. The product thus formed does not include a relatively low density core or central portion.
In accordance with the present invention, it has been found that the surface density and physical characteristics of a relatively low density consolidated mat can be altered in a post-press operation to harden, smooth, and densify the surfaces thereof when at least the surface fibers of the mat are contacted with urea. During the post-press operation, at a temperature of at least 525.degree. F., the urea reacts to stiffen and strengthen the cellulosic substrate, such as a low density fiberboard, to provide the necessary strength for the intended purpose of the substrate while retaining a relatively low density, lightweight central core of the material. In this manner, the resulting post-pressed substrate is very lightweight while having strength and stiffness properties equal to or better than much heavier boards.